Kerr effect read-out system for an optical memory

ABSTRACT

The magnetic state of a material exhibiting the Kerr effect is monitored by a system which directs a polarized light beam normal to the surface of the material and which includes a polarization sensitive element capable of directing light over two different paths depending on the polarization of the light and a polarization shifter located between the polarization sensitive element and the magnetic material.

United Sta1 [72] Inventor Di Che FOREIGN PATENTS [21] A 1 No g a f1,192,743 5/1965 Germany 356/114 PP 1 [22] Filed July 15, 1970 OTHERREFERENCES 45 patented 7, 1971 D. Chen, et al.,J. Appl. Phys. Vol. 36,#3 3/65 pp. l26l [73] Assignee Honeywell Inc. 1263' Minneapolis, Minn.H. J. Williams, et al., J. Appl. Phys. Vol. 28, #10, 10/57 pp.1181l18454 KERR EFFECT READ-OUT SYSTEM FOR AN "f:,:?:; ;:f g

OPTIFAL P Attorneys- Lamont B. Koontz and Omund R. Dahle 7 Claims, 4Drawing Flgs.

[52] US. Cl 356/118,

250/225, 340/174.1 MO, 350/151, 356/114 [51] Int. Cl G01n 21/44,

G1 1b 1 1/10 [50] Field of Search 356/118,

l l 1,1;350/15ll340/174" MO; 346/74 MT; ABSTRACT: The magnetic state ofa material exhibiting the 250/225 Kerr effect is monitored by a systemwhich directs a polarized 156121:12:2221212212322115:21512115522152:11531:; UNITED STATES PATENTSlight over two different paths depending on the polarization of3,401,590 9/1968 Massey 356 14 the light and a polarization shifterlocated between the 3,284,785 11/1966 Kornei ..340/l74.l (MO) ensitiveelement and the magnetic material,

A B FOCUSI NG AND 1 I 222112115 C 1 1 g i 24 25 23 i 22 I I i l l g L LI :1 J I i d I POLARIZED POLARIZATION POLARI GHT i SENSITIVE smri riMAGNET'C SOURCE ELEMENT MED'UM DETECTOR PAIEIIIEII DEC 7 I971 SHEET 1 0F2 FIG. PRIOR ART i I4 II i I POLARIZED FOCUSING a MAGNETIC LIGHTPOSITIONING MEDIUM SOURCE W APPARATUS ANALYZER V l6 1 I OETEcTOR FIG. 2

FOCUSING AND A B POSITIONING C I I APPARATUS I 22 20 I 24 I 25 23 g 1 2|1 J i 3 I l w l l l I POLARIZED POLARIZATION POLARIZATION MAGN m LIGHTsENsITIvE SH |FTER J SOURCE ELEMENT zD ETECTOR INVIiNlOR DI CHEN (PM A(9% A TTOR/VE Y.

KERR EFFECT READ-OUT SYSTEM FOR AN OPTICAL MEMORY BACKGROUND OF THEINVENTION Magnetic materials which have two stable magnetic states andwhich exhibit Kerr rotations for these states which are equal inmagnitude but opposite in sign can be used as a memory medium for amagneto-optic memory. Some magnetic materials have a preferredmagnetization direction which is parallel to the surface of the materialand therefore exhibit a longitudinal or transverse magneto-optic Kerreffect. One such material is permalloy film. Other materials such asmanganese bismuth film and the orthoferrites have a preferredmagnetization direction which is normal to the surface of the material,and therefore these materials exhibit a polar magneto-optic effect.

In an optical memory using a magnetic material, information can bestored by the use of Curie point writing and can be read out bymonitoring either the Farady or the Kerr rotation. In such an opticalmemory system, it is desirable to monitor the Kerr rotation with anincident beam which is essentially normal to the surface of the magneticmaterial, since only one set of light positioning and focusing means isthen required. Since the materials necessary to provide a lightpositioning apparatus are quite expensive, a reduction of one-half inthe amount of optics required is highly desirable. A reduction in theamount of optics required is highly desirable. A reduction in the amountof optics also allows for a corresponding reduction in the size of anoptical memory system. In addition, it has been shown that for magneticmaterials exhibiting a polar magneto-optic effect, maximum rotation isobtained when the incident light beam is normal to the surface of thematerial.

Prior art optical readout systems FIG. 1 which monitor the Kerr rotationwith an incident beam essentially normal to the surface of the magneticmedium 11 make use of a semitransparent or beam splitter mirror 12placed between the light source 13 and the focusing and lightpositioning means 14 with the reflecting surfaces inclined to thedirection of the beam. An analyzer 15 which passes a higher intensity oflight for one direction of rotation than for the opposite direction ofrotation is located between the beam splitter and the detector 16. Sincethe beam splitter mirror 12 partially reflects and partially transmitslight, it results in a reduction in the intensity of the light whichreaches the detector. If the beam splitter 12 reflects one-half of theincident light beam, the maximum intensity of the beam reaching thedetector 16 is one-fourth of the intensity of the light supplied by thepolarized light source 13.

SUMMARY OF THE INVENTION In the present invention a polarizationsensitive element, which directs light over two different pathsdepending upon the polarization of the light, replaces thesemitransparent mirror used in the prior art. Such polarizationsensitive element can be a polarizing beam splitter such as a Glan,Glan-Thomson, or Nicol polarizer with an exit window on the side, or aRochon or Wallaston prism. The polarization sensitive element isoriented so as to pass light which is polarized in the direction of thepolarization of the incident beam, and therefore the beam passesunattenuated to the magnetic medium, where it is rotated and reflectedback to the polarization sensitive element. The component of thereflected beam which is polarized in the direction of polarization ofthe incident beam passes through the element and back to the sourcewhile the Kerr component of the light is directed to the detector. TheKerr rotation for positive and negative magnetization of the medium is+0 and 6 respectively, and therefore if I, is the electric fieldamplitude of the reflected light, the amplitude of the light reachingthe detector is I Sin6 for positive magnetization and l,,,Sin(0 fornegative magnetization. The detector measures the intensity of the lightwhich it receives, which is the square of the amplitude of the light andtherefore the detector can differentiate between zero magnetization,

which creates no rotation of polarization and Kerr component, and aregion of nonzero magnetization having a nonzero Kerr component.However, since Sin 0 =Sin (0 the detector is unable to differentiatebetween a region of positive magnetization and one of negativemagnetization, and therefore the replacement of the mirror beam splitterby a polarization sensitive means does not by itself enhance the opticalreadout system, but in fact makes it inoperable.

The present invention further includes a polarization shifter such as aFaraday cell which is placed in the path of the beam between thepolarization sensitive element and the magnetic medium. The polarizationshifter provides an additional rotation each time the beam passesthrough it so that the Kerr component will differ in magnitude, therebyallowing the detector to distinguish between positive and negativemagnetization of the medium.

DESCRIPTION OF THE DRAWING FIG. 1 is a schematic drawing of a prior artoptical readout system employing a beam splitter mirror.

FIG. 2 is a schematic drawing of the preferred embodiment of the presentinvention.

FIG. 3 is a diagram of the polarization vectors of the incident andreflected beams at various positions in an optical readout systememploying a polarization sensitive element but no polarization shifter.

FIG. 4 is a diagram of the polarization vectors of the incident andreflected beams at various positions in an optical readout systememploying a polarization sensitive element and a polarization shifter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 2, apolarized light source 20 provides light beam 21 which is directedessentially normal to the surface of the magnetic medium 22. Focusingand light positioning means 23 make it possible to address variousregions of the magnetic memory medium 22 with the light beam 21. Apolarization sensitive element 24 is located in the path of the lightbeam 21 between the source 20 and the light positioning and focusingmeans 23. The polarization sensitive element 24 is oriented so thatlight having a polarization direction the same as that of the incidentbeam travels over one path while light polarized in another direction isdirected over a second path. A polarization shifter 25, such as aFaraday cell, is located between the polarization sensitive element 24and the focusing and deflection system 23. A light detector 26 ispositioned to receive the Kerr component of the reflected light.

Since the polarization sensitive element 24 is oriented to transmit thecomponent of light polarized in the direction of the polarization of theincident beam, the incident beam passes unattenuated to the surface ofthe magnetic medium 22. Upon reflection, the beam returns to thepolarization sensitive element 24, where the entire Kerr component isdirected to the detector. It can be seen that the detector in thepresent invention receives four times the amount of power of that of theprior art system.

In order for the detector to be able to differentiate between the regionof positive magnetization and one of negative magnetization, it isnecessary to provide a polarization shifter between the polarizationsensitive element and the focusing and deflection means. Thepolarization shifter, which could comprise a Faraday cell, provides anadditional rotation 0,, with each passage of the beam through theshifter. The polarization direction of the reflected beam is thereforebiased by 249 so that the total rotation for positive magnetization ofthe medium is 20 +0 and the total rotation for negative magnetization ofthe medium is 26 -0 The components of light reflected by thepolarization sensitive element are l Sin (20 +0 and I Sin (ZO -6respectively, and therefore the detector is able to distinguish betweenpositive and negative magnetization of the medium. If 0 =6 /2, there isno signal at the detector for a beam reflected from negativemagnetization regions and a maximum signal for a positive magnetizationregion. However, it is not necessary to make =6 /2 if a certain amountof background signal or intensity can be tolerated. In this case, 0should be greater than 6 /2 The necessity of polarization shifter 25 isillustrated by the diagrams of FIGS. 3 and 4. FIG. 3 shows thepolarization vectors of the incident and reflect beam at variouspositions when 0 =0, or in other words, when no polarization shifter ispresent in a system using a polarization sensitive element 24. It can beseen the entire incident beam passes unattenuated through thepolarization sensitive element 24 to the magnetic medium 22. Thepolarization vector is rotated by +0 or 6,,. depending upon themagnetization of the magnetic medium. Upon reaching the polarizationsensitive element 24, the beam is split, with that component I,,,Cos (+6or I,,,Cos (6 having a polarization the same as the incident vectorbeing directed back toward the light source 20 over path A. The Kerrcomponent I,,,Sin (+0 or I,,,Sin (O,,.) is directed over path D. It canbe seen that while the polarization vector at D differs in sign, it hasthe same magnitude for both positive and negative magnetization. Sincethe detector measures intensity of light, which is the square of theamplitude of the light, the detector is unable to distinguish betweenpositive and negative magnetization when no polarization shifter 25 isused.

FIG. 4 illustrates the polarization vectors of the incident andvreflected beams when a polarization shifter is used. In order to obtainmaximum contrast between positive and negative mag netization, thepolarization shifter 25 provides an additional rotation 6 0,12. As inFIG. 3, the incident beam (A,) passes through the polarization sensitiveelement unattenuated (B,). Passage through the polarization shiftercauses a rotation of the vector 6,, (C,). The vector is then furtherrotated by +0 or 0 by the magnetic medium 22 (C The reflected beampasses through polarization shifter 25 and is rotated by an additional6,, (B Since 6,,=6 /2, the resultant shift in polarization for negativemagnetization is zero, the Kerr component (D is zero, and the entirebeam (A passes through the polarization sensitive element 24 toward thelight source 20. The Kerr component (D for positive magnetization isnonzero, and therefore the detector 26 is able to determine themagnetization of the magnetic medium.

In one system utilizing manganese bismuth film having a 6, 2, a glassrod two inches long made of optical quality glass such as BK7 is used asthe polarization shifter supplying a bias of 1 rotation when a 600 Gaussfield is applied. The polarization sensitive element is a Glan polarizerwith an exit window on the side. A laser provides the light source,allowing the system to be used for Curie point writing as well asoptical readout.

It is to be understood that this invention has been disclosed withreference to a preferred embodiment, and it is possible to make changesin form and detail without departing from the spirit and scope of thisinvention.

What is claimed is:

I. A system for detecting, by the magneto optic Kerr effect, the stateof a magneto-optic material having two stable states, where themagneto-optic Kerr rotations for the two stable states are equal inmagnitude but opposite in sign, the system comprising:

a light beam source for projecting along a path polarized light beamhaving a first polarization direction,

the magneto-optic material positioned to receive the light beam atessentially normal incidence and to rotate the polarization direction ofthe light beam and reflect the light beam back toward the light beamsource over essentially the same path,

a polarization sensitive element located between the light beam sourceand the magneto-optic material, the polarization sensitive element beingoriented to pass essentially unattenuated the light beam projectedtoward the magneto-optic material, to direct over a first path acomponent of the light beam reflected from the magnetooptic materialwhich has the first polarization direction, and to direct over a secondpath a component of the light beam having a polarization directiondifferent from the first polarization direction,

light detector means positioned to receive the component directed overthe second path,

polarization shifter means for rotating the direction of polarization ofthe light beam by an angle sufficient to cause the component of thelight beam received by the detector means to have a first amplitude ifthe magnetooptic material is in one of the two stable states and asecond different amplitude if the magneto-optic material is in the otherof the two stable states, said polarization shifter means locatedbetween the polarization sensitive element and the magneto-opticmaterial, and

focusing and light positioning means located between the polarizationshifter means and the magneto-optic material.

2. The system of claim 1 wherein the polarization sensitive element is apolarizing beam splitter.

3. The system of claim 1 wherein the magneto-optic material is aferromagnetic medium.

4. The system of claim 3 wherein the ferromagnetic medium has apreferred magnetization direction normal to the surface of the material.

5. The system of claim 4 wherein the ferromagnetic medium is manganesebismuth film.

6. The system of claim 1 wherein the means for rotating the polarizationof the light beam is a Faraday cell.

7. The system of claim 1 wherein the rotation of the polarization for asingle passage through the means for rotating the polarization isessentially equal to one-half the Kerr rotation of the magneto-opticmaterial.

1. A system for detecting, by the magneto-optic Kerr effect, the stateof a magneto-optic material having two stable states, where themagneto-optic Kerr rotations for the two stable states are equal inmagnitude but opposite in sign, the system comprising: a light beamsource for projecting along a path polarized light beam having a firstpolarization direction, the magneto-optic material positioned to receivethe light beam at essentially normal incidence and to rotate thepolarization direction of the light beam and reflect the light beam backtoward the light beam source over essentially the same path, apolarization sensitive element located between the light beam source andthe magneto-optic material, the polarization sensitive element beingoriented to pass essentially unattenuated the light beam projectedtoward the magneto-optic material, to direct over a first path acomponent of the light beam reflected from the magneto-optic materialwhich has the first polarization direction, and to direct over a secondpath a component of the light beam having a polarization directiondifferent from the first polarization direction, light detector meanspositioned to receive the component directed over the second path,polarization shifter means for rotating the direction of polarization ofthe light beam by an angle sufficient to cause the component of thelight beam received by the detector means to have a first amplitude ifthe magneto-optic material is in one of the two stable states and asecond different amplitude if the magneto-optic material is in the otherof the two stable states, said polarization shifter means locatedbetween the polarization sensitive element and the magneto-opticmaterial, and focusing and light positioning means located between thepolarization shifter means and the magneto-optic material.
 2. The systemof claim 1 wherein the polarization sensitive element is a polarizingbeam splitter.
 3. The system of claim 1 wherein the magneto-opticmaterial is a ferromagnetic medium.
 4. The system of claim 3 wherein theferromagnetic medium has a preferred magnetization direction normal tothe surface of the material.
 5. The system of claim 4 wherein theferromagnetic medium is manganese bismuth film.
 6. The system of claim 1wherein the means for rotating the polarization of the light beam is aFaraday cell.
 7. The system of claim 1 wherein the rotation of thepolarization for a single passage through the means for rotating thepolarization is essentially equal to one-half the Kerr rotation of themagneto-optic material.